This invention relates to a centrifugal clutch and, in particular, to a centrifugal clutch of the type which includes: a clutch drum; a plurality of clutch shoes arranged within the clutch drum in such a way as to be radially moveable and adapted to be moved radially outwards by centrifugal force to come into frictional contact with the inner peripheral surface of the clutch drum; and a spring for biasing the clutch shoes radially inwards so as to normally keep them away from the inner peripheral surface of the clutch drum.
Generally speaking, in a centrifugal clutch of this type, a pair of swingable clutch shoes, each of which is rotatably supported at one end, are pulled toward the center of the clutch drum by an extension coil spring, as shown in FIG. 3, or annular coil springs arranged within circumferential grooves (not shown) formed in the clutch shoes. When applied to a portable working machine like a chain saw, such a centrifugal clutch involves, for example, the following problems: in the rotating speed range in which the centrifugal force acting on the extension coil springs is in excess of the mounting load of these springs, each extension coil spring suffers deformation due to the centrifugal force and thereby affects the movement of the clutch shoes, so that a smooth movement of the clutch shoes cannot be obtained. Further, a sliding-friction movement is caused in the extension coil springs, which accelerates the wear of these springs, resulting in a premature breakage of the extension coil springs and impairment of their durability.